What is STASH?
STASH is the Unified Model's storage handling and diagnostic system. It is designed to cope with the many different configurations that the UM can be used in, but still provide output in a consistent and standard way.
Prognostic and Diagnostic Fields
The UM considers variables (or fields) to be of two different types, prognostic or diagnostic. Prognostic fields are ones which the model must have values for, prior to each timestep, as the equations of motion the model solves require these fields (these are fields such as specific humidity or potential temperature) so they must exist in the model start dump. Diagnostic fields are all other fields that are derrived from prognostic ones, and as such the model does not need to have prior values for these. Ancillary files (such as emissions, SSTs etc) contain prognostic fields.
From a user's perspective, STASH is used to output fields during the run, and from this point of view it does not matter if these are prognostic or diagnostic fields.
